The production of gas from the partial oxidation of hydrocarbonaceous fuels, especially coal in the form of anthracite, bituminous, lignite or peat, has been utilized for a considerable period of time and has recently undergone significant improvements due to the increased energy demand. In these methods, the hydrocarbonaceous fuels are reacted with a reactive oxygen-containing gas, such as air or oxygen, optionally in the presence of a temperature control moderator in a gasification zone to obtain the hot partial oxidation gas. In addition to coal, various other hydrocarbonaceous fuels are suitable as feedstocks for the gasification process.
The term "hydrocarbonaceous" as used herein to describe various suitable feedstocks is intended to include gaseous, liquid, and solid hydrocarbons, carbonaceous materials, and mixtures thereof. In fact, substantially any combustible carbon-containing organic material, or slurries thereof, may be included within the definition of the term "hydrocarbonaceous". For example, there are (1) pumpable slurries of solid carbonaceous fuels, such as particulate carbon dispersed in a vaporizable liquid carrier, such as water, liquid hydrocarbon fuel, and mixtures thereof; and (2) gas-liquid-solid dispersions, such as atomized liquid hydrocarbon fuel and particulate carbon dispersed in a temperature moderating gas.
The term "liquid hydrocarbon," as used herein to describe suitable liquid feedstocks, is intended to include various materials, such as liquefied petroleum gas, petroleum distillates and residua, gasoline, naphtha, kerosene, crude petroleum, asphalt, gas oil, residual oil, tar-sand oil and shale oil, coal derived oil, aromatic hydrocarbons (such as benzene, toluene, xylene fractions), coal tar, cycle gas oil from fluid-catalytic-cracking operations, furfural extract of coker gas oil, and mixtures thereof.
"Gaseous hydrocarbon fuels," as used herein to describe suitable gaseous feedstocks, include methane, ethane, propane, butane, pentane, natural gas, coke-oven gas, refinery gas, acetylene tail gas, ethylene off-gas, and mixtures thereof. Solid, gaseous, and liquid feeds may be mixed and used simultaneously; and these may include paraffinic, olefinic, acetylenic, naphthenic, and aromatic compounds in any proportion.
Also included within the definition of the term "hydrocarbonaceous" are oxygenated hydrocarbonaceous organic materials including carbohydrates, cellulosic materials, aldehydes, organic acids, alcohols, ketones, oxygenated fuel oil, waste liquids and by-products from chemical processes containing oxygenated hydrocarbonaceous organic materials, and mixtures thereof.
Depending upon its intended use, the partial oxidation gas produced in a gasification process may be referred to as synthesis gas, reducing gas, or fuel gas. The generic terms "partial oxidation gas" and "producer gas" will be used herein to collectively refer to all of these potentialities.
In a typical gasification process, a raw producer gas stream, substantially comprising H.sub.2, CO, and at least one gas from the group H.sub.2 O, CO.sub.2, H.sub.2 S, COS, CH.sub.4, NH.sub.3, N.sub.2, Ar and often containing entrained solids, i.e., particulate carbon and ash, is produced by partial oxidation of a hydrocarbonaceous fuel with a free-oxygen containing gas, optionally in the presence of a temperature moderator, in the reaction zone of an unpacked free-flow noncatalytic partial-oxidation gas generator. The atomic ratio of free oxygen to carbon in the fuel (O/C ratio), will typically be in the range of about 0.6 to 1.6, and preferably about 0.8 to 1.4. The reaction time will typically be in the range of about 1 to 10 seconds, and preferably about 2 to 6 seconds. When steam is used as the temperature moderator the steam-to-fuel weight ratio in the reaction zone will typically be in the range of about 0.1 to 5, and preferably about 0.2 to 0.7.
The raw producer gas stream will typically exit from the reaction zone at a temperature in the range of about 1,300.degree. to 3,000.degree. F., and more typically in the range of about 2,000.degree. to 2,800.degree. F., and at a pressure typically in the range of about 1 to about 250 atmospheres, and more typically in the range of about 15 to about 150 atmospheres.
The typical gas generator comprises a vertical cylindrically shaped steel pressure vessel lined with refractory, such as disclosed in coassigned U.S. Pat. No. 2,809,104. Typically a quench drum for cooling the hot effluent stream of gas from the reaction zone to a temperature in the range of about 300.degree. to 600.degree. F. by direct contact with water will also be incorporated into the gas generator. This arrangement is also disclosed in U.S. Pat. No. 2,809,104. At least a portion of the entrained solids, i.e., particulate carbon and ash, are removed from the process gas stream by the turbulent quench water and a pumpable dispersion of particulate carbon and water containing about 0.1 to 4.0 wt. % particulate solids is produced in the quench tank incorporated into the gasification reactor. Any remaining entrained solids, water vapor, and unwanted gaseous contaminants are removed from the process gas stream in additional operations.
While the composition of the raw gas stream leaving the gas generator will vary depending upon, among other things, the type of hydrocarbonaceous fuel used and process conditions, a typical partial oxidation gas emerging from the gasification reactor will have the following mole percent compositions on a dry basis: H.sub.2 O 6 to 29, CO 20 to 57, CO.sub.2 2 to 30, CH.sub.4 nil to 25, H.sub.2 S nil to 2, COS nil to 0.1, NH.sub.3 nil to 0.1, N.sub.2 nil to 60, and Ar nil to 0.5. Trace amounts of cyanides may also be present. Water will typically be present in the gas in the range of about 1 to 75 mole percent. Particulate carbon will typically be present in the range of about 0.5 to 20 weight percent (basis carbon content in the original feed). Ash and other particulate matter may also be present.
The hot partial oxidation gas which is withdrawn from the gasification zone and subjected to cleansing operations to rid it of various contaminants which are formed or liberated from the hydrocarbonaceous fuel during the gasification step. These contaminants can readily become environmental pollutants if not properly treated. For example, unwanted contaminants often found in the hot partial oxidation gas include water vapor, hydrogen sulfide, carbonyl sulfide, ammonia, cyanides, various halogens and particulates in the form of carbon, ash, and coal, as well as trace metals. The extent of the contaminants in the partial oxidation gas is often determined by the type of hydrocarbonaceous fuel, particularly when coal is employed, the particular gasification process utilized, as well as, the operating conditions. In any event, the disposal and control of these pollutants are major problems in the gasification processes which must be satisfactorily handled in order to make gasification a viable process without suffering attendant pollution problems.
Of the variety of methods employed to remove contaminants from the partial oxidation gas emerging from the gasifier, many involve the use of a scrubbing tower. In the typical scrubbing tower, producer gas emerging from the gasifier is bubbled through a volume of water contained in the tower. After the bubbling, an appreciable amount of the particulate contaminants remain in the water. These particulates initially form a dispersion in the water and over time and as the water cools settle to the bottom of the tower where they can be removed through a blowdown or other outlet port. The water will often also contain some trace metals and halogens. The water will likewise often contain very small levels of contaminants, like ammonia, hydrogen sulfides, carbonyl sulfides, and cyanides, that are at least somewhat soluble in the water. These levels, however, will be very small due to the temperature of the water and the process pressures. The procedure in which the partial oxidation gas is brought in contact with water to remove contaminants is referred to as "scrubbing."
The water used for the scrubbing operation becomes what is commonly known as "dirty water," since it is contaminated with particulates. This dirty water may be subjected to a variety of steps which may include the stripping of the water to remove the small amounts of hydrogen sulfide, carbonyl sulfide, and ammonia, and also solvent extraction to remove the small amounts of cyanides and the other inorganic anions, such as the halogens.
After bubbling, the partial oxidation gas emerges from the water. However, the gas emerging from the water is not substantially free of contaminants. Substantially all of the ammonia, hydrogen sulfide, carbonyl sulfide, and cyanides initially present in the gas stream entering the scrubber are still present in the gas emerging after bubbling from the water. Additionally, the emerging gas will contain a significant amount of water vapor. Of particular concern in the present invention is the presence of ammonia and water vapor. Among other problems that may occur if these contaminants are not substantially removed, the water can cause problems with a downstream flare if not removed in time and ammonia can interfere with process steps in which sulfur containing contaminants are removed.
Also present in the emerging gas are residual levels of particulate contaminants. As such, prior art processes have been designed to further reduce ammonia and particulate levels present in the gas after the initial bubbling.
In prior art processes, additional removal of particulates is often achieved by placing a series of vertically stacked and offset trays above the water in the scrubber. Water is provided to the top of these trays and is channeled to the bottom of the trays where it joins the volume of water contained in the scrubber bottom. As the emerging gas containing residual particulates comes in contact with the water, additional scrubbing occurs with the result being that the additional amounts of particulates are carried with the water to the bottom of the scrubber for subsequent removal.
The efficiency of this particulate removal process is directly related to the steam pressure in scrubber head space above the water. When the water temperature is high, the amount of steam, and therefore the steam pressure, in the overhead is also high. Conversely, when the water temperature is low, steam pressure and concomitantly scrubbing efficiency are also low.
From the foregoing, it would appear obvious that the solution to maintaining peak scrubbing efficiency in the overhead is to maintain the water temperatures as high as possible. Unfortunately, this solution is not without its own problems.
While increased scrubbing efficiency is directly related to increased water temperature, particulate settling rate is inversely related to water temperature. The consequences of these two adverse relationships is demonstrated as follows. The continuous addition of gas containing particulates to the scrubber dictates that at some point the particulates must be removed. Preferably, removal of contaminants is achieved without completely shutting down the scrubber. As previously disclosed, this is commonly achieved by means of a blowdown located at the bottom of the scrubber.
It should be evident that peak removal efficiency is achieved with higher settling rates. As the settling rate increases, the blowdown volume will increasingly be comprised primarily of particulates with the amount of water removed being reduced. Increased settling rates therefore have the additional benefit of reducing the amount of make-up water that must be added.
While increased settling rates are desirable, they, as disclosed, often can not be achieved in prior art processes without cooling the water in the scrubber. But as also disclosed, the cooling of the water detrimentally affects the scrubbing efficiency in the scrubber overhead.
Increased settling rates are also frustrated by turbulence. Producer gas entering the volume of water generates considerable turbulence and agitation. The scrubbing efficiency in the body of water is due in some part to this turbulence. However, the turbulence adversely affects the rate of settling for the particulates once they are separated from the gas.
In some prior art processes, the problem of turbulence and its effects on settling is remedied by the constant dumping of the bottoms of the scrubber to a low pressure settler. While this modification might solve the problem of turbulence and its effect on particulate settling, it, also, is not without its faults. In particular, this modification dictates that make-up water be added to the scrubbing tower at very high rates as the bottoms of the scrubber will contain a substantial portion of water. Additionally, some portion, albeit a very small portion, of the partial oxidation gas that has not had sufficient time to bubble to the surface of the water is also dumped. Such a modification is therefore inefficient because of the increased water requirements and the lower resulting yields of partial oxidation gas. As such, this and other modifications evident in the prior art have not been entirely acceptable.
It would therefore be desirable to discover a quenching and scrubbing system wherein separation and removal of the particulate contaminants generated during the gasification and entrained in the production gases is improved. In particular, it would be desirable to minimize the amount of makeup water that is necessary.
Another problem with prior art gasification processes relates to the removal of ammonia and cyanides, particularly ammonia as the concentration of ammonia typically greatly exceeds the cyanide concentration.
As disclosed, the partial oxidation gas exiting a scrubbing operation still contains substantially all of the ammonia, hydrogen sulfide, carbonyl sulfide, and cyanides initially present. It will also contain a considerable amount of water vapor. For almost all, if not all, of the intended uses of the partial oxidation gas, these contaminants must be removed. Removal of the water vapor, ammonia, and the cyanides, typically in the form of hydrogen cyanide, is advantageously achieved first as these contaminants will either condense at higher temperatures and pressures or dissolve in water at higher temperatures and pressures.
Typically, the hot partial oxidation gas exiting the scrubbing operation is passed through a series of heat exchangers and knockout drums or their equivalents to reduce the temperature of the gas stream, thereby effectuating the removal of water and ammonia as condensate. As disclosed, complete removal, or substantially complete removal, of water and ammonia is desired as the presence of these materials has an adverse effect on downstream operations, notable flaring and sulfur removal. As such, an inordinate number of heat exchangers and knockout drums, or their equivalents, have typically been used to help ensure complete removal of ammonia and water vapor. Unfortunately, undesirable levels of ammonia are often still present after these series of cooling and washing steps.
Therefore, it would be desirable to discover a cooling and washing system that more efficiently removed water vapor and ammonia from wet hot partial oxidation gas. In particular, a system that did not require an excessive number of heat exchangers and knockout drums, or their equivalents, would be desirable.
In accordance with one aspect of the invention, a scrubbing tower and high pressure settler assembly comprising a dip tube, a bottom portion, a top portion, and a high pressure settler is provided. The dip tube transports partial oxidation gas from an injection point on the exterior of the scrubbing tower into a volume of water contained in the bottom portion of the scrubbing tower. A blowdown port capable of removing particulate matter is connected to the bottom portion of the assembly. A series of trays is provided in the top portion of the scrubbing tower. The top portion of the scrubbing tower also has inlet ports for receiving water and an outlet port for releasing the scrubbed partial oxidation gas. The use of the high pressure settler facilitates higher particulate settling rates as well as higher scrubbing efficiencies.
In accordance with another aspect of the invention, a process for removing particulates from partial oxidation gas in a scrubbing tower and high pressure settler assembly is provided. The process comprises: bubbling partial oxidation gas containing particulates through water in the scrubbing tower under conditions sufficient to separate particulate matter from the partial oxidation gas; removing the separated particulates via a high pressure settler; passing the separated gas and any residual particulates through a series of trays; and providing water to the series of trays such that the gas emerging from the trays is substantially free of particulates; and recovering the gas emerging from the series of trays.